It is often necessary to transfer information between elements in a digital computer system. In some situations, information is transmitted from a transmitting circuit to a receiving circuit across a data line. The information is typically transmitted as a voltage potential for a predetermined time interval. The voltage potential of the transmitted signal may be affected by attenuation due to parasitics, signal noise due to coupling with other signals, and power supply noise due to active switching of circuit elements in the digital computer system. As a result, the characteristics of the voltage potential received at the receiving circuit may not be ideal.
A typical receiving circuit compares the voltage potential of the transmitted signal against a reference voltage potential. If the voltage potential of the transmitted signal is greater than the reference voltage potential, a binary one is indicated by the receiving circuit. Alternatively, if the voltage potential of the input signal is less than the reference voltage potential, a binary zero is indicated by the receiving circuit.
Typically, the reference voltage potential, also known in the art as a “switching reference voltage,” is generated on a printed circuit board. A voltage divider connected to a power supply on the printed circuit board may be used to generate the reference voltage potential. The reference voltage line that transmits the reference voltage potential has parasitics that include impedances from the printed circuit board trace, the integrated circuit package, the integrated circuit, and the connections between the three elements.
The reference voltage potential is commonly charge-coupled to a power supply on the integrated circuit. The parasitics on the reference voltage line and the charge-coupling form a low-pass filter. The low-pass filter helps remove unwanted high-frequency noise in the reference voltage potential.
FIG. 1 shows a typical reference voltage potential circuit (100). A voltage divider including resistors R5 (102) and R2 (104) are connected between power supplies VDD (101) and VSS (103). The resistors' (102, 104) values determine the reference voltage potential (105). The resistors (102, 104) and power supplies (101, 103) may reside on a printed circuit board. The power supplies (101, 103) and reference voltage potential (105) are transmitted to an integrated circuit (110). The lines that transmit the voltage potentials of the power supplies (101, 103) and reference voltage potential (105) to the integrated circuit (110) have impedances represented by Z5 (130), Z7 (150), and Z3 (106), respectively. The voltage potentials affected by the impedances (130, 150, 106) are transmitted to the integrated circuit (110) by lines (132, 152, 107), respectively.
The integrated circuit (110) and the package (not shown) that allows the integrated circuit to be mounted to the printed circuit board, add additional impedances to the lines (132, 152, 107). The additional impedances are represented by Z6 (140), Z8 (160), and Z4 (108), respectively. The reference voltage potential (109) on the integrated circuit (110) is charge-coupled to the power supplies (134, 154) on the integrated circuit (110) through capacitors C1 (120) and C2 (122), respectively.
FIG. 2 shows a timing diagram (200) of a reference voltage potential circuit. The power supply difference signal (201) shows the absolute difference between power supplies VDD and VSS, for example, the absolute difference between power supplies (134, 154) shown in FIG. 1. A reference voltage signal (203) shows a reference voltage potential resulting from a voltage divider circuit, for example, the voltage divider using R5 (102) and R2 (104) shown in FIG. 1. The reference voltage signal (203) is dependent on the power supply difference signal (201).
In FIG. 2, clock signals (205, 207) are transmitted from a transmitting circuit to a receiving circuit. The clock signals (205, 207) are affected by attenuation due to parasitics, signal noise due to coupling with other signals, and/or power supply noise due to active switching of circuit elements in the digital computer system. The reference voltage signal (203) is not depend on the clock signals (205, 207), is not centered with respect to the clock signals (205, 207), and is not dependent on the receiving circuit's semiconductor process, temperature, voltage, and/or input termination impedance.